Contact structure for an electric circuit interrupter



y 4, 1963 s. R. SMITH, JR 3,089,936

CONTACT STRUCTURE FOR AN ELECTRIC CIRCUIT INTERRUPTER Filed Feb. 23,1960 v Inventor:

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United States Patent ()1 3,689,936 Patented May 14, 1%63 ice spasms(IGNTAQT STRUCTURE FUR AN ELECTRTQ Cl R CUlT llN'liEilRUlTElt Sidney R.Smith, the, toclthridge, Mass, assignor to General Electric Company, acorporation of New York Filed Feb. 23, 1M0, Ser. No. llhljtl l3 Qiairns.(@l. 200-44 4) This invention relates to contact structure for anelectric circuit interrupter and, more particularly, to contactstructure which is especially, though not exclusively, suited for use incircuit interrupters of the vacuum type.

The usual vacuum-type circuit interrupter comprises a pair of separablecontacts disposed within an evacuated chamber. Circuit interruption isinitiated by separating these contacts to establish an arc. Assumingthat the circuit is an alternating current circuit, the arc maintainsitself until about the time a natural current Zero is reached, afterwhich the arc is prevented from reigniting by the high dielectricstrength of the vacuum.

It has been recognized heretofore that the interrupting capacity of suchan interrupter can be materially increased by moving the terminals ofthe are at high speed along the surfaces of the contacts or adjacentstructure. Such movement tends to minimize the amount of metallic vaporsgenerated from the contacts or adjacent structure by the arc and tendsalso to increase the degree of diffusion of the vapors that aregenerated. These factors enable the vacuum to recover its dielectricstrength at an increased rate after a current zero and thus render thevacuum more capable of preventing reestablishment of the arc during thiscritical interval.

In types of interrupters other than those of the vacuum type, high speedarc-movement also facilitates circuit interruption by cooling the arccolumn, decreasing contactheating and vaporization, and promoting anincreased rate of dielectric recovery.

Prior schemes for moving the arc terminals over the contact suriaceshave usually relied upon a magnetic field generated by one or more coilsconnected in circuit with the contacts. These coils must generally berather massive and bulky and are also difiicult to free of adsorbedgases, which are distinct disadvantages, particularly for a vacuumswitch construction.

Thus, an object of my invention is to provide structurally-simple,easily-manufactured, and compact means for driving an are at high speedalong the surfaces of a contact or similar structure.

Another object is to incorporate such arc-driving means into the contactstructure itself and in such a manner that the arc is driven in agenerally circular path from the instant it is first initiated.

Still another object is to construct contact structure of this characerin such a manner that butt-type circuitclosing engagement between thetwo contacts is made over a relatively large area and at a plurality ofdifferent points.

In carrying out my invention in one form, I provide a pair of coactingbutt contacts relatively movable from a position of engagement to aposition of disengagement to establish a circuit interrupting arctherebetween. At least one of the contacts is a cup-shaped membercomprising a body portion and a generally cylindrical flange projectinglaterally from said body portion toward the other contact. The flangeterminates at its free end in a generally annular contact-making surfacethat extends about the periphery of the cup-shaped member and faces theother contact. The other contact has a contactmalcing surface alignedwith the contact-making surface of the first contact and buttingthereagainst when the contacts are engaged. Arc-motion in acircumferential direction along the contact-making surfaces is effectedby means comprising a plurality of slots formed in the flange of thecup-shaped member. Each of these slots intersects the contact-makingsurface of the cup-shaped member and extends from this contact-makingsurface toward said body portion via a pair disposed at an acute angleto said contact-making surface. These slots force any current flowing toan arc terminal located at any angular position on the contact-makingsurface to have a net component extending generally circumferentiallywith respect to the flange in the region of the arc. This netcircurnferentially-extending component forms with the arc a loop circuitthat has a magnetic effect that acts to drive the arc in acircumferential direction about the contact-making surface.

For a better understanding of my invention, reference may be had to thefollowing description taken in conjunction with the accompanyingdrawing, wherein:

FIG. 1 is a sectional view of a vacuum circuit interrupter comprisingcontact structure embodying one form of my invention.

FIG. 2 is an en arged side-elevational view of the contact structureshown in MG. 1.

P16 2:: is a view taken along the line Za-Za of FIG. 2.

FIG. 3 illustrates a modified form of contact structure for use in aninterrupter of the type shown in FIG. 1.

FIG. 4 illustrates still another modified form of contact structure foruse in an interrupter of the type shown in PEG. 1.

Referring now to the interrupter of FIG. 1, there is shown a highlyevacuated envelope 13 comprising a casing 11 of suitable insulatingmaterial and a pair of metallic end caps 12 and 13 closing off the endsof the casing. Suitable seals 14 are provided between the end caps andthe casing to provide vacuum-tight joints at these points.

Located within the envelope in is a pair of separable contacts 17 and 18shown in their engaged, or closedcircuit, position. The upper contact 17is a stationary Contact suitably secured to a conductive rod 17a, whichat its upper end is integrally united to the upper end cap 32. The lowercontact 18 is a movable contact joined to a conductive operating rod 18awhich is suitably mounted for vertical movement. The operating rod 18aprojects through an opening in the lower end cap If and a flexiblemetallic bellows 2%; provides a seal about the rod 13a to allow forvertical movement of the rod without impairing the vacuum inside theenvelope lit. As shown in FIG. 1, the bellows 2t} is secured by means ofsuitable sealed joints at its respective opposite ends to the operatingrod 18a and the end cap 13.

Coupled to the lower end of the operating rod 130, suitable actuatingmeans (not shown) is provided. This actuating means is capable ofdriving the contact 18 downwardly out of engagement with the contact 17so as to open the interrupter and is also capable of returning thecontact in to its illustrated position so as to close the interrupter. Acircuit-opening operation will soon be explained in greater detail.

Each of the contacts 27 and is is of a cup-shaped configuration andcomprises a body portion 25 of disc form having at its periphery acylindrical flange 27 laterally protecting therefrom toward the othercontact. The flange 2'7 of each contact terminates at its free end in agenerally annular contachmaking surface 29 that extends about theperiphery of the cup-shaped member 27 and faces the other contact. Thesecontact-making surfaces 29 on each contact butt against each other incontact-making relationship when the circuit interrupter is closed, asshown in MG. 1.

The flange 27 of each contact has a series of slots 33 provided thereinto form fingers 34- located between the slots. As will be apparent fromFIG. 2, in each contact these slots intersect the contact-making surface29 and extend from said contact-making surface 29 toward the bodyportion 25 via a path disposed at an acute angle to the contact-makingsurface 29. In view of this acute angular relationship, the slots may bethought of as extending from the contact-mal ing surface 29 in agenerally circumferential direction with respect to flange 27. In eachcontact, each of these generally circumferentially extending slotsterminates only after it has extended from its mouth at thecontact-making surface as to a point near the angular position of themouth of an adjacent slot. Preferably, the adjacent slots of eachcontact angularly overlap each other as is shown in KG. 2. As will beapparent from FIG. 2a, each of these slots 33 extends through the entirethickness of the flange 2'7, i.e., from the outer periphery to the innerperiphery of the flange. In addition, the slots 33 in both contactsextend circumferentially away from their mouth at the contact-makingsurface 29 in generally the same angular direction, e.g., in FIG. 2 bothsets of slots 33 extend circumferentially away from their mouth at thecontact-making surface 29 in a clockwise direction,

as considered from the bottom of the contact structure I Assume now thatthe circuit is to be interrupted and to this end, the contact 13 isdriven downwardly to establish a circuit-interrupting arc. The are isinitiated between the two contact-making areas 29 on the particular setof fingers which were last to part. Substantially all of the currentflowing between the conductor 17a and the upper arc terminal isconcentrated in the particular finger 3d of contact 17 on which the arcwas initiated, and, likewise, substantially all of the current flowingbetween conductor 1dr: and the lower arc terminal is concentrated in theparticular finger of contact 13 on which the arc was initiated. Becauseof the generally circumferential manner in which the slots 33 extend,the current flowing through these two fingers 3:4- is required to followa path which has a net component extending generally circumferentiallywith respect to the flange 27 in the region of the arc, as isillustrated by the dotdash lines 35 of FIG. 2. As a result of thiscircumferentially-extending configuration of the current path, there isa circuit present which has its arms extending effectivelycircumferentially with respect to the arc interconnecting the arms. Aloop circuit has a magnetic effect which tends in a well-known manner tolengthen the loop, and because of the circumferential disposition of thearms of the loop, this magnetic effect is a circumferentially-actingforce which acts in FIG. 2 to drive the arc circumferentially to theleft. When an arc terminal has moved to the left hand end of the finger3 this circumferentially-acting force drives the arc terminal across theslot 33 to the next finger 34. The current flowing to the arc terminalis then concentrated in this next finger 34, and because of theconfiguration of this finger, there is a new circumferentially-actingloop circuit which continues motion of the are around the contactperiphery. For each of the fingers 34, there is a net circumferentialforce component on the are acting in the same angular direction, and, asa result, circumferential motion of the arc continues at high speeduntil the arc is finally extinguished. As viewed from the bottom of FIG.2, this circumferential arc-motion would be in a counterclockwisedirection. if the arc is not extinguished before one revolution iscompleted, circumferential motion will continue repetitively about thecontact periphery until the arc is extinguished. It will be understoodthat the fingers are sufiiciently close together to enable the arc to bedriven across any slot which separates the fingers.

The above described movement of the arc is advantageous in that itlessens the amount of contact material that will be vaporized by the arcand also increases the degree of difiusion of the vapors that aregenerated. This enables the vacuum to recover its dielectric strength atan increased rate and thereby improves ability to preventre-establishment of the are after a current zero, thus increasing theinterrupting capacity of the interrupter. A particular advantage of mycontact arrangement shown in FIGS. 1 and 2 is thatcircumferentially-acting forces are present to begin circumferentialmotion of the arc 'rorn the instant of its initiation. No delay isrequired to allow time for the arc to be driven onto the arc-rotatingregion of the contact.

in the embodiment of FIGS. 1 and 2, the slots are of a straight-lineconfiguration. This straight-line configuration is advantageous in thatit enables the slots to be easily and quickly produced, as by a simplesawing or milling operation. Curved slots may be used if desired, butthese are generally more di ficult to form than the straight-line slotsshown in FIGS. 1 and 2. Whether the slots are straight or curved,however, the cup should be slotted from its contact-making surface 29inward, and the slot configuration should be such that the current pathextending between the body portion 25 of the contact and an arc terminallocated at substantially any angular point on the contact-making surfaceZfi has a net component extending generally circumferentially withrespect to the flange 27 in the vicinity of the arc. In addition, theslot configuration should be such that this net circumferentialcomponent extends from the arc in the same angular direction forsubstantially all angular positions of the are on the peripheral regionof the contact, so that motion of the arc terminal is continued in asingle angular direction. By the term angular direction, usedhereinabove, is meant a clockwise or a counterclockwise directionrelative to the central region of the contact.

In the contact structure of FIGS. 1 and 2, the circumferentially-actingforce component tending to drive the are around the contact-makingsurface 29 exists only so long as one of the arc terminals is attachedto one of the fingers 34. If both arc terminals were to be locatedradially inwardly of the fingers 34, then there would be nocircumferentially acting force on the arc. In my contact arrangement,there is a radially-acting force component which continuously biases thearc in a radially-outward direction, thereby preventing it fromwandering into a region where there is no circumferentiallyacting forcecomponent and forcing it to remain in the outer peripheral region wherethere is an effective circumferentially-acting force component. Thisradiallyoutwardly-acting force component that biases the arc toward theouter periphery of the contact structure resalts from the fact that theloop circuit exending through the contact extends radially, i.e.,through the body portion 25, as well as circumferentially, as a resultof the cup-shaped configuration of the contacts.

It will be apparent that the contact arrangement of the presentinvention requires no special coils or magnets for effecting the desiredarc rotation and is therefore more compact than prior arrangements thathave utilized such supplementary devices. A feature that furthercoutributes to the compactness of my contact arrangement is the factthat the fingers 3- which are responsible for thecircumferentially-extending disposition of the current paths leading toand from the arc, extend from the body of each contact in a generallyaxial direction relative to the longitudinal axis of the envelope iii.By extending in an axial direction rather than in a radial direction, anenvelope of considerably smaller diameter is needed for enclosing thecontacts with the required radial clearance. A somewhat increased lengthof envelope is required, but increases in length can be eflected withcon siderably more economy than increases in diameter of the envelope.

If lower speeds of are motion can be tolerated, only a single one of thecontacts need be provided with areimpelling means. For example, as shownin FIG. 3, the lower contact can be formed as a planar disc abutting thecontact-making surface 29 of the upper contact about its outerperiphery. The lower contact, though unslotted, will still provide aradially-acting loop component which tends to hold the are on the outerperipheral region of the contact structure where thecircunrfer'entially-acting force component is effective.

Another advantage of the disclosed contact arrangement is that contactis made over a relatively large area at a plurality of different points.In this regard, the fingers 34 are capable of yielding slightly in anaxial direction to allow firm contact to be made at a plurality ofdifferent fingers on each contact. By making contact at a plurality ofpoints, I am able to lessen the amount of current flowing through anysingle point when the contacts are closed, and this reduced currentlessens the tendency for contact welding to occur at any given pointduring the occurrence of current surges.

Although I have shown the slots 33 as being uniformlyspaced about theperiphery of the contact, it is to be understood that non-uniformperipheral spacing could instead be utilized. Non-uniform spacingbetween the slots would be advantageous in lessening the tendency of thecontacts to bounce temporarily out of engagement in response to theimpact produced by initial contactclosing engagement. This follows fromthe (fact that nonuniform spacing of the slots would result in fingers34 of differing mass and hence, differing natural frequency. Because oftheir differing natural frequencies, there would be less tendency forany bounce that would occur to result in all of the fingers separatingfrom the mating contact simultaneously. Some of the fingers would alwaystend to be in contact with the mating contact, thereby minimizing anytendency to are during this interval.

In some cases, it may be advantageous to make contact and initiate arcson a portion of the contact structure apart from the arc-runningsurface. One way in which this can be accomplished is illustrated in theembodiment of FIG. 4. Here each contact is provided with a slottedflange 27 corresponding to the flange '27 of FIGS. 1 and 2, but inaddition, each contact has a contact-making ring concentric .with theflange 27 and located internally thereof. The contact-making ring 40 ofeach contact extends alongside the slotted flange 27 from the bodyportion 25 to a location slightly beyond the end of flange 27 so thatcon tact is made at the outer surface 4 1 of the ring 4t} rather than onthe flange 27. When the contacts are separated, an arc is initiatedbetween the contact-making rings 40 and is driven radially outward ontothe arc-running surfaces 29 of the flanges 27 by theradially-outwardly-acting magnetic effect resulting from the loopcircuit 4-2. When the arc reaches the arc-running surface 29", it isdriven in a circumferential direction in the same manner as described inconnection with FIGS. 1 and 2.

A gap 44 is provided between the contact-making ring 40 and thearc-running flange 27 to insure that current flowing to and from an arcterminal on the arc-running surface 29 is forced to flow through thefingers 34 of the flange 27 rather than being allowed to follow a paththrough the rings 40 that would bypass the lingers 34 and impair theirability to produce arc-movement circumferentially about the surface 29;

Another advantage of the disclosed contact arrangements is that theyreadily lend themselves to the establishment of a symmetrical electricfield in the region of the arcing gap, i.e., a field which is generallysymmetrical with respect to a reference plane which bisects the arcinggap between the fully-open contacts and extends perpendicular to thelongitudinal axes of the contact supporting rods. This symmetry followsfrom the fact that each set of the disclosed contacts themselves are ofgenerally the same shape, particularly in FIGS. 1 and 4, and aredisposed generally symmetrical with respect to the above referenceplane. With regard to this general matter, it should be noted thatarcing gaps in general have a lower breakdown strength when subjected tovoltage of one polarity than when subjected to voltage of an oppositepolarity. The more non-symmetrical is the electric field in the regionof the gap, the more pronounced is this polarity effect. By providing agenerally symmetrical field, I am able to minimize this polarity effect.As a result, my interrupter is not subjected to the unduly prolongedarcing that could result from low dielectric strength dur ing alternatehalf-cycles.

For protecting the insulation of a vacuum interrupter from the build-upof a metallic coating thereon as a result of arc-generated vaporscondensing thereon, it is customary to provide a vapor-condensing shieldbetween the arcing gap of the interrupter and the protected insulatingsurface. Such a shield is shown at 50 and comprises a cylindricalmetallic member disposed between the contacts 17, 13 and the internalsurface of insulating casing 11. This shield preferably corresponds to asimilar shield shown and claimed in Patent No. 2,892,911, Crouch,assigned to the assignee of the present invention, and is thereforeelectrically isolated from both contacts and ground.

While I have described my invention panticul-arly with regard to avacuum circuit interrupter, it will be apparent that it also may beadvantageously embodied in other types of interrupters. For example, thedisclosed contact structure could be used in the type of interrupterthat has its contacts disposed in a chamber filled with an arcextinguishing medium, which may, if desired, be pressurized.

While I have shown and described particular embodiments of my invention,it will be obvious to those skilled in the art that various changes andmodifications may be made without departing from my invention in itsbroader aspects, and I, therefore, intend in the appended claims tocover all such changes and modifications as fall within the true spiritand scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In an electric circuit interrupter, a pair of coacting butt contactsrelatively movable from a position of engagement to a position ofdisengagement to establish a circuitinterrupting arc therebetween, atleast one of said contacts being a cup-shaped member comprising a bodyportion and a flange projecting later-ally from said body portion towardsaid other contact, said flange terminating at its free end in agenerally annular contact-making surface extending about the peripheryof said cup-shaped member and facing said other contact, said othercontact having a contact-making surface generally aligned wtih thecontactmaking surface of said first contact and butting thereagainstwhen said contacts are engaged, said contact-making surfaces partinglast during a circuit-interrupting operation to initiate said arebetween said contact-making surfaces, means for producing arc-motion ina circumferen tial direction along said contact-making surfacescomprising a plurality of slots in said flange extending through theentire radial thickness of the flange, each of said slots intersectingsaid contact-making surface and extending from said contact-makingsurface toward said body portion via a path disposed at an acute angleto said contactmaking surface.

2. The interrupter of claim 1 in which said other contact is acup-shaped member comprising a body por tion and a flange projectinglaterally therefrom With the contact-making surface thereof located atthe free end of said flange, the flange of said second contact beingprovided with a plurality of slots, each intersecting the contact-makingsurface of said second contact and extending from said lattercontact-making surface toward the body portion of said second contactvia a path disposed at an acute angle to said latter contact-makingaosaase surface, the slots of both contacts extending from theirintersection with the contact-making surfaces of their respectivecontacts in generally the same angular direction.

3. The arrangement of claim 1 in which said slots have a straight-lineconfiguration.

4. The interrupter of claim 1 in which each of said slots has a mouthlocated at the intersection of said slot and said contact-mfiing surfaceand in which each of said slots extends from its mouth generallycircumferentially of said cup-shaped contact at least to a point closelyadjacent the angular position of the mouth of an adjacent slot.

5. The interrupter of claim 4 in which at least some of said slotsextend into angularly overlapping relationship with their respectiveadjacent slots.

6. The interrupter of claim 4 in which the spacing between some of theslots is difierent from the spacing between others of the slots so thatfingers of varying mass are formed between the slots.

7. In an electric circuit interrupter, a pair of coacting butt contactsrelatively movable from a position of engagement to a position ofdisengagement to establish a circuit interrupting arc therebetween, atleast one of said contacts being a cup-shaped member comprising a bodyportion and a flange projecting laterally from said body portion towardsaid other contact, said flange terminating at its free end in agenerally annular contactmaking surface extending about the periphery ofsaid cup-shaped member and facing said other contact, said other contacthaving a contact-making surface generally aligned with thecontact-making surface of said first contact and butting thereagainstwhen said contacts are engaged, means for producing arc-motion in acircumferential direction along said contact-making surfaces Comprisinga plurality of slots in said flange extending through the entire radialthickness of said flange, said slots intersecting said contact-makingsurface and extending toward said body portion, the slot configurationbeing such that the current path extending between said body portion andan arc terminal located at any point on said contact-making surfaceforms with said arc a loop circuit having a magnetic effect exerting anet component of force on the are acting generally circumferentiallywith respect to said flange, said net tangential force component actingin the same angular direction for substantially all angular positions ofsaid arc on said contact-making surface and thereby causing said are totravel rapidly in one angular direction along said contact-makingsurface, the portions of said contact-making surface on either side ofany slot therein being sufficiently close together to enable saidcircumferentially-acting component of force to drive said arc acrosssaid slot.

8. In an electric circuit interrupter, a pair of coacting butt contactsrelatively movable from a position of engagement to a position ofdisengagement to establish a circuit-interrupting arc therebetween, atleast one of said contacts being a cup-shaped member comprising a bodyportion and a flange projecting from said body portion toward said othercontact, said flange terminating at its free end in a generally annulararc-running surface extending about the periphery of said cup-shapedmember and facing said other Contact, an annular arc-running surfaceelectrically connected to said other contact and generally aligned withthe arc-running surface on said first contact, means for producing anarc between said arc-running surfaces when said contacts are separated,means for producing arc motion in a circumferential direction along saidarc-running surfaces comprising a plurality of slots in said flangeextending through the entire radial thickness of said flange, each ofsaid slots intersecting the arc-running surface of said first contactand extending from said latter arc-running surface toward the bodyportion of said first contact via a path disposed at an acute angle tosaid latter arc-running surface.

9. The interrupter of claim 8 in which each of said slots has a mouthlocated at the intersection of said slot and the arc-running surface ofsaid first contact, and in which each of said slots extends from itsmouth generally circumferentially of said cup-shaped contact at least toa point closely adjacent the angular position of the mouth of anadjacent slot.

10. The interrupter of claim 8 in which said slots have a straight lineconfiguration.

11. The arrangement of claim 8 in which said cupshaped contact includesarc-initiating means for initiating circuit interrupting arcs radiallyinwardly of said arcrunning surface and arc-impelling means for drivingsaid arc radially outward onto the arc-running surface of saidcup-shaped contact, said arc-impelling means comprising aradially-outwardly-acting loop circuit that includes a conductor forcarrying current to and from said cupshaped contact member locatedradially-inwardly of said are initiating means.

12. The arrangement of claim 8 in which said cupshaped contact includescontact-making structure disposed radially inwardly of said flange andextending alongside said slots from the body portion of said contacttoward said are running region, insulating means disposed between saidcontact-making structure and said flange for forcing current flowing toan arc terminal located on said arc-running surface to follow a paththrough the slotted portion of said flange, and arc-impelling means fordriving arcs initiated on said contact-making structure radially outwardonto said arc-running surface comprising a radially-outwardly actingloop circuit that includes a conductor for carrying current to and fromsaid contact-making structure located radially inward of saidcontact-making structure.

13. In an electric circuit interrupter, a pair of co-acting contactsrelatively movable from a position of engagement to a position ofdisengagement to establish a circuit-interrupting arc therebetween, atleast one of said contacts comprising a tubular portion projectingtoward said other contact, said tubular portion terminating at its freeend in a generally annular arc-running surface extending about theperiphery of said tubular portion and facing said other contact, anannnular arc-running surface electrically connected to said othercontact and generally ali ned with the arc-running surface on said firstcontact, means for producing an arc between said arc-running surfaceswhen said contacts are separated, means for producing arc motion in acircumferential direction along said arc-running surfaces comprising aplurality of slots in said tubular portion extending through the entireradial thickness of said tubular portion, each of said slotsintersecting the arc-running surface of said first contact and extendingfrom said latter arc-running surface generally longitudinally of saidtubular port-ion via a path disposed at an acute angle to said latterarc-running surface.

References Cited in the file of this patent UNITED STATES PATENTS716,848 Read Dec. 23, 1902 2,140,378 Biermanns et al Dec. 13, 19382,421,267 Huber May 27, 1947 2,900,476 Reece Aug. 18, 1959 2,949,520Schneider Aug. 16, 1960 2,976,382 Lee Mar. 21, 1961 FOREIGN PATENTS474,185 Great Britain Oct. 27, 1937 580,720 Germany July 15, 1933556,719 Belgium May 15, 1957 787,846 Great Britain Dec. 18, 1957

1. IN AN ELECTRIC CIRCUIT INTERRUPTER, A PAIR OF COACTING BUTT CONTACTS RELATIVELY MOVABLE FROM A POSITION OF ENGAGEMENT TO A POSITION OF DISENGAGEMENT TO ESTABLISH A CIRCUITINTERRUPTING ARC THEREBETWEEN, AT LEAST ONE OF SAID CONTACTS BEING A CUP-SHAPED MEMBER COMPRISING A BODY PORTION AND A FLANGE PROJECTING LATERALLY FROM SAID BODY PORTION TOWARD SAID OTHER CONTACT, SAID FLANGE TERMINATING AT ITS FREE END IN A GENERALLY ANNULAR CONTACT-MAKING SURFACE EXTENDING ABOUT THE PERIPHERY OF SAID CUP-SHAPED MEMBER AND FACING SAID OTHER CONTACT, SAID OTHER CONTACT HAVING A CONTACT-MAKING SURFACE GENERALLY ALIGNED WITH THE CONTACTMAKING SURFACE OF SAID FIRST CONTACT AND BUTTING THEREAGAINST WHEN SAID CONTACTS ARE ENGAGED, SAID CONTACT-MAKING SURFACES PARTING LAST DURING A CIRCUIT-INTERRUPTING OPERATION TO INITIATE SAID ARC BETWEEN SAID CONTACT-MAKING SURFACES, MEANS FOR PRODUCING ARC-MOTION IN A CIRCUMFERENTIAL DIRECTION ALONG SAID CONTACT-MAKING SURFACES COMPRISING A PLURALITY OF SLOTS IN SAID FLANGE EXTENDING THROUGH THE ENTIRE RADIAL THICKNESS OF THE FLANGE, EACH OF SAID SLOTS INTERSECTING SAID CONTACT-MAKING SURFACE AND EXTENDING FROM SAID CONTACT-MAKING SURFACE TOWARD SAID BODY PORTION VIA A PATH DISPOSED AT AN ACUTE ANGLE TO SAID CONTACTMAKING SURFACE. 